1. Field of the Invention
This invention relates to offshore drilling rigs, and more particularly, to an improved shock absorber mechanism and method for use on the existing leg structure of offshore jack-up rigs.
2. General Background and Prior Art
In common oilfield procedures, offshore jack-up drilling rigs are often moved from one drilling and production location on the ocean floor to another location. Inherent to being moved, the legs of the drilling rigs encounter varying degrees of shock due to their being raised from and lowered onto the ocean floor. As the rigs are moved, their legs are lowered to and raised from the ocean floor while under the influence of the ocean environment, such as heave, roll, pitch and the like. For example, when a site has been chosen for the drilling rig, the legs of the rig are lowered into place onto the ocean floor. As the legs are being lowered, the forces acting upon the rig, i.e., heave, wave, pitch, roll, etc., cause the legs to rock, sway, pitch, and bob up and down, sometimes contacting the ocean floor at the base of the legs. The resulting shock to the legs of the rigs as they make contact with the ocean floor, have, in some cases, been severe and in many instances resulted in damage, and at times destruction, to the leg structure and the mechanisms for elevating and lowering the legs. Accordingly, such activities are undertaken only during relatively calm seas. In turn, the waiting time, for calm seas, can increase the costs associated with the rig operation.
There are shock absorber devices which attempt to overcome the problems of excessive shock forces to the legs when positioning the rigs. Some of the known solutions provide for compression members which absorb the shock due to impact on the ocean floor. These systems, however, can only be used in relatively calm waters and are complex structures which are not readily accessible for repairs and maintenance. These compression devices are costly and require that a large compression surface area be provided for absorption of high impact.
Further, one device in particular, which is currently being used in the industry, consists of a long lightweight skirt which is fixed around the periphery of the drilling rig, extending from the top of the platform into a depth below the bottom of the platform. Within the housing of the skirt are located air jets, which are activated when the platform is being raised or lowered. As the legs of the rig are lowered into place, the air jets draw the ocean water up into the skirt to attempt to maintain a level position on the water surface, inhibiting swaying, bobbing and the like. However, this device also is only effective in relatively calm water conditions, and further, from its positioning on the periphery of the platform, the skirt itself is exposed to damage from approaching vessels or floating objects, i.e., supply barges or floating debris. While reducing roll and pitch, it does very little to reduce vertical heave, the major cause of impact forces.
Still, other shock absorbing devices for offshore structures address problems associated with the forces caused by waves, wind, and ocean currents. However, these devices are designed for use after an offshore structure has been anchored onto the ocean floor. They do not relieve the problems of vertical impact when the structure is being raised or lowered from its anchored position. The legs structure and its mechanism for elevating and lowering the platform is not protected from the vertical impact forces.
It is therefore an object of the present invention to provide a system of absorbing shock to the legs of an offshore rig while the legs are being positioned at a pre-determined location.
It is an object of the present invention to provide a resilient shock absorbing system which is flexible enough to substantially counteract damaging forces acting on the legs of an offshore rig caused by their impact with an ocean floor.
It is a further object of the present invention to provide a simple shock absorbing device, which is more over adapted for quick operation and maintenance, and which can be used in rough ocean conditions.
These and other objects of the invention will be more apparent from the following description of the present invention.